Hot melt adhesives are typically thermoplastic adhesives that are solid at room temperature, but and must be melted prior to use. Hot melt adhesives may be used for coating substrates during the sealing of packages, and may be used during such diverse activities as building construction, shoe manufacturing, and bookbinding. In addition, many uses exist in the assembly of automobile parts, electronics, electrical equipment, appliances, electrical components, furniture and other widely ranging manufacturing processes. The common forms of hot melt adhesives include, for example, polyamide adhesives, pressure sensitive adhesives (PSA), ethyl vinyl acetate (EVA), polyurethane reactive adhesives (PUR), and other animal based adhesives. The present invention relates most specifically to PUR adhesives, however, its various aspects are applicable to other types of meltable thermoplastic materials as used in many circumstances, such as those mentioned above.
PUR adhesives cure in the presence of ambient moisture. Typically, this type of adhesive may be supplied in a solid slug form, such as in the shape of a solid cylinder of material. A cylindrical melting hopper is often used to contain this slug of adhesive and a melting grid is typically disposed at the bottom of the hopper to gradually melt the material. The melted material flows from a hopper outlet, which communicates directly or indirectly with the melting grid, and a pump is typically used to dispense the melted material, for example, from a dispensing gun. After PUR material is melted, but before it is dispensed during a manufacturing operation, the material must be maintained in a substantially dry environment to prevent curing. Moisture in the air space contained at the top of a melting hopper can lead to undesirable curing of the material within this space. Gas injection or blanket or purge systems have been attempted to maintain this upper air space in a dry state, such as by using injected dry nitrogen gas. A dangerous condition can exist, however, if the injection of dry gas is sustained into an over pressure condition. Also, if the gas purge system does not have an effective warning system to indicate a low gas pressure condition, then the dry environment of the hopper could be compromised without the knowledge of the operator.
Other problems in the hot melt dispensing art relate to the melting grid used at the bottom of the hopper. These melting grids must communicate either directly or indirectly with an outlet for the melted thermoplastic material. The outlet must also remain unobstructed by the solid, unmelted hot melt material. In many cases, it is most desirable to have the slug of thermoplastic material disposed directly on the melting grid and over the outlet. However, this general type of design can lead to obstruction of the outlet. While various grid structures have been used to help ensure that the outlet remains unobstructed, there remains a need for improvements in this area of the hot melt unit, for example, to ease maintenance and clean up associated with the melting grid.
Various additional problems exist in this area of technology. Therefore, it would be desirable to provide a thermoplastic material melting and dispensing apparatus which incorporates various improvements related to the above concerns as well as other problems associated with this technology.